1. Field of the Invention
The present invention relates generally to an optical pickup device for use in an optical information recording/reproducing apparatus that records and/or reproduces signals from an optical information medium such as an optical disc.
2. Description of the Related Art
For a recording/reproducing apparatus for recording/reproducing information on an optical disc, loaded therein, such as an optical video disc, a digital audio disc, and so on, a focus servo and a tracking servo are essential for always accurately converging light beams for writing and reading information to a pit train or the like formed spirally or concentrically on a recording surface of the optical disc. The focus servo performs a positional control for an objective lens, used to irradiate a pit train on the optical disc with light beams, in an optical axis direction so as to reduce a focus error, i.e., an error of the position of the objective lens in the optical axis direction with respect to the focus position of the objective lens. The tracking servo performs a positional control for the position of the objective lens, used to irradiate a pit train on the optical disc with a light beams, with respect to a recording track in a radial direction of the optical disc, so as to reduce a tracking error, i.e., an error of the objective lens with respect to the pit train recording track position.
FIG. 1 illustrates a conventional optical pickup device using the astigmatism method.
A laser beam from a semiconductor laser 1 is transformed into a parallel laser beam by a collimator lens 2, passes through a polarizing beam splitter 3, and is converged by an objective lens 4 toward an optical disc 5 to form a light spot onto a pit train on an information recording surface of the optical disc 5.
Light reflected from the optical disc 5 is converged by the objective lens 4 and directed by a beam splitter 3 to a detecting lens 7. A converged light beam formed by the detecting lens 7 passes through a cylindrical lens 8, serving as an astigmatism generating element, to form a spot image near the center xe2x80x98Oxe2x80x99 of a light receiving surface of a quadrant photodetector 9 having four light receiving surface areas (elements) divided by two orthogonal line segments. The cylindrical lens 8 irradiates the quadrant photodetector 9 with a light spot SP in the shape of true circle as illustrated in FIG. 2A when the laser beam is converged on the recording surface of the optical disc 5 in focus, and an elliptic light spot SP, extending in an orthogonal direction of the elements as illustrated in FIG. 2B or 2C when the converged laser beam is out of focus on the recording surface of the optical disc 5 (FIG. 2B illustrates the light spot SP when the objective lens 4 is too far from the optical disc 5, while FIG. 2C illustrates the light spot SP when the objective lens 4 is too near the optical disc 5), thus generating so-called astigmatism.
The quadrant photodetector 9 opto-electrically transduces the light spot irradiated to the four light receiving surface areas into respective electric signals which are supplied to a focus error detecting circuit 12. The focus error detecting circuit 12 generates a focus error signal (FES) based on the electric signals supplied from the quadrant photodetector 9 and supplies the focus error signal to an actuator driver circuit 13. The actuator driver circuit 13 supplies a focusing driving signal to an actuator 15. The actuator 15 moves the objective lens 4 in response to the focusing driving signal in the optical axis direction.
The focus error detecting circuit 12, as illustrated in FIG. 3, is connected to the quadrant photodetector 9, where the quadrant photodetector 9 is composed of four detecting elements DET1 to DET4 in first to fourth quadrants which are located adjacent to each other with two orthogonal division lines L1 and L2 interposed therebetween and which are independent of each other. The quadrant photodetector 9 is positioned such that the division line L2 is in parallel with a tangential direction with respect to the extending direction of the recording track, and the other division line L1 is in parallel with the radial direction of the same. Respective opto-electrically transduced outputs from the elements DET1 and DET3, symmetric with respect to the center xe2x80x98Oxe2x80x99 of the light receiving surface of the quadrant photodetector 9, are added by an adder 22, while respective opto-electrically transduced outputs from the elements DET2 and DET4, also symmetric with respect to the center xe2x80x98Oxe2x80x99 of the light receiving surface, are added by an adder 21, and outputs from the respective adders 21 and 22 are supplied to a differential amplifier 23. The differential amplifier 23 calculates the difference between the supplied signals, and outputs a signal indicative of the difference therebetween as a focus error signal (FES).
As described above, in the conventional focus error detecting circuit 12, the outputs of the quadrant photodetector 9 are added by the adders 21 and 22, respectively, and the differential amplifier 23 calculates the difference between the outputs of the adders 21 and 22 to generate a focus error component. In this event, when the light beam is in focus, the light spot in the shape of true circle as illustrated in FIG. 2A is formed on the quadrant photodetector 9, where a spot intensity distribution is symmetric with respect to the center xe2x80x98Oxe2x80x99 of the light receiving surface of the quadrant photodetector 9, i.e., symmetric in the tangential direction and in the radial direction, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are equal to each other, with the focus error component being calculated to be xe2x80x9czeroxe2x80x9d. On the other hand, when the light beam is out of focus, i.e., an elliptic light spot extending in a diagonal direction as illustrated in FIG. 2B or 2C is formed on the quadrant photodetector 9, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are different from each other. Thus, the focus error component output from the differential amplifier 23 exhibits a value corresponding to the focus error. Specifically, assuming that the references designated to the elements of the quadrant photodetector 9 represent the outputs thereof, the focus error signal FES is expressed by the following equation:
FES=(DET1+DET3)xe2x88x92(DET2+DET4)
Since the objective lenses of conventional CD players have a small numerical aperture and a large focal depth, slight noise if appeared on a focus error signal (FES) would be negligible as a focus error. In a case where information is read from an optical disc having lands and grooves, such as a DVD-RAM, however, the numerical aperture of the objective lens becomes larger and the focal depth thereof becomes shallower, so that the influence of noise contained in the focus error signal on the focus servo of the objective lens becomes greater.
In conventional optical pickup apparatuses which read information from an optical disc having pre-grooves, therefore, the focus servo system cannot follow up noise on an FES, thus raising problems like oscillation of the focus servo circuit and heating of the actuator.
Accordingly, the present invention has been made in view of the problem mentioned above, and thus an object thereof is to provide an optical pickup apparatus capable of adequately eliminating noise from a focus error signal that is generated at the time a light spot crosses a track or groove in the astigmatism method, particularly, the noise component that is originated from a further astigmatism of the optical system or birefringence of the substrate of an optical disc.
An optical pickup device according to the present invention comprises an objective lens unit having an objective lens, support means for supporting the objective lens and an objective-lens driving mechanism for driving the objective lens in a radial direction and a focusing direction of an optical disc in such a way that a light beam is focused on an information recording surface of the optical disc; a main unit having an optical irradiation system which includes a light source for emitting a light beam and guides the light beam to the objective lens, and an optical detection system which includes photosensing means and guides reflected light from the information recording surface to the photosensing means via the objective lens; and tilt-position-adjusting means for supporting the objective lens unit on the main unit and tilting an optical axis of the objective lens with respect to an optical axis of the optical irradiation system in such a way as to minimize an undesirable astigmatism given by the optical disc and optical elements in the optical irradiation system and the optical detection system.
In the optical pickup device having the structure above mentioned, said tilt-position-adjusting means includes a supporting structure for fixing the objective lens unit at least two points onto said main unit after the adjustment of position for tilting said objective lens unit is performed on the main unit using at least one screw at one of the points.
In the optical pickup device having the structure above mentioned, said supporting structure is that said objective lens unit is fixed on said main unit at three points which do not lie in a straight line, at one point of which said objective lens unit is anchoringly in contact with the main unit, and at remaining two points of which the adjustments of position for said objective lens in the tilting are performed with two screws, so that the optical axis of the objective lens is tilted toward one of a pair of regions including the vertical angles of the first to fourth quadrants defined by the tangential and radial directions of the optical disc all around the optical axis of the optical irradiation system on the main unit.
In the optical pickup device having the structure above mentioned, said supporting structure is that said objective lens unit is fixed on said main unit at two points, at one point of which said objective lens unit is anchoringly in contact with the main unit, and at a remaining points of which the adjustments of position for said objective lens in the tilting are performed with a screw, so that the optical axis of the objective lens is tilted toward one of a pair of regions including the vertical angles of the first to fourth quadrants defined by the tangential and radial directions of the optical disc all around the optical axis of the optical irradiation system on the main unit.
Another optical pickup device according to the present invention comprises:
an objective lens;
support means for supporting said optical lens;
an objective-lens driving mechanism for driving said objective lens in a radial direction and a focusing direction of an optical disc in such a way that a light beam is focused on an information recording surface of said optical disc;
an optical irradiation system which includes at least one light source for emitting a light beam and guides said light beam to said objective lens;
an optical detection system which includes photosensing means and guides reflected light from said information recording surface to said photosensing means via said objective lens; and
shift-position-adjusting means for shifting an optical axis of a light beam irradiated from said light source with respect to an optical axis of said optical irradiation system so that an optical axis of said objective lens is tilted with respect to an optical axis of said optical irradiation system so as to minimize an undesirable astigmatism caused from said optical disc and optical elements in said optical irradiation system and said optical detection system.
In the optical pickup device having the structure above mentioned, said shift-position-adjusting means including screwing mechanisms for moving the optical axis of the light source in directions corresponding to the tangential and radial directions of the optical disc from the optical axis of the optical irradiation system individually.
In the optical pickup device having the structure above mentioned, said shift-position-adjusting means including a screwing mechanism for moving the optical axis of the light source in a direction which does not correspond to the tangential and radial directions of the optical disc from the optical axis of the optical irradiation system.
In the optical pickup device having the structure above mentioned, said optical irradiation system including; a collimator lens converting a diverging light beam irradiated from the semiconductor laser to a parallel light; and a complex prism guiding the parallel light beam to the objective lens while shaping a cross-section of the light beam and having a partial function of the optical detection system as to guide a reflected light from the optical disc to said optical detection system.
In the optical pickup device having the structure above mentioned, said photosensing means which comprises; an astigmatism generating element giving astigmatism the reflected light; and a quarter-split photosensor having at least four elements arranged in a point symmetrical to one another with respect to the center of a light-receiving surface.
In the optical pickup device having the structure above mentioned, the optical pickup device further comprises a diffraction-grating element which forms two light spots from plus and minus first-order light beams that are irradiated in a point symmetrical fashion around a 0-th order light spot in a middle distance between the two light spots.
In the optical pickup device having the structure above mentioned, the optical pickup device further comprises rotational adjusting means which rotates the diffraction-grating element with respect to the optical axis of the optical irradiation system in such a manner that the radial component of the distance between the centers of the two light spots becomes xc2xd of a track pitch of the optical disc.
In the optical pickup device having the structure above mentioned, said astigmatism generating element is one of selected form a cylindrical lens, hologram element and parallel transparent plate.
The optical pickup apparatus of the present invention adjusts the angular position of the objective lens itself to allow a light beam to obliquely enter the objective lens, providing an image height which produces an astigmatism to nearly minimize the undesirable astigmatism of optical elements in the optical irradiation system and optical detection system and an optical disc, so that a noise component originated from the astigmatism or the birefringence of the substrate of the optical disc can be adequately eliminated from the focus error signal.